At least one embodiment of the invention relates to the use of non-fired refractory products in the form of pressed molded bodies or as a moldable or plastic mass or ramming castable to create refractory masonry—referred to hereinafter also as a liner or lining or lining—in a large-volume industrial furnace, in which cement, lime, magnesia or doloma are fired in an oxidizing or essentially oxidizing atmosphere. At least one embodiment of the invention furthermore relates to a large-volume industrial furnace resulting from such use. “Essentially oxidizing atmosphere” is supposed to mean that the masonry is impacted, during operation of an industrial furnace, predominantly with an oxidizing atmosphere over time, and only part of the time also with a neutral atmosphere.
Cement, lime, magnesia or doloma are fired in large-volume rotary kilns or shaft kilns, in an oxidizing or essentially oxidizing atmosphere; the lining of these kilns generally consists of fired refractory products.
In EP 1 599 697 A1, it is proposed to use non-fired bricks of refractory material to line large-volume industrial furnaces in the cement, lime, dolomite, and magnesite industry, which bricks contain carbon carriers in the form of graphite or soot and furthermore carbon resulting from carbonaceous binder in the fire-side surface region of the masonry of the lining. The bricks can consist of different refractory materials and can also demonstrate spinels, aside from MgO, among other things.
The carbon carriers, but particularly the graphite, impart undesirably high heat conductivity to the known bricks, so that measures must be taken for protection of the metallic kiln jacket that is always present. Furthermore, the graphite makes the refractory product more expensive. Also, graphite produces a smooth surface of the bricks, which hinders the brick-laying persons during the creation of a furnace lining, due to the risk of slipping. Furthermore, when an oxidizing atmosphere acts on the carbon sequestration of the brick for an extended period of time, the binding effect of the structure is already weakened at relatively low temperatures or actually lost, despite the presence of antioxidants that are supposed to protect the carbon against oxidation, and thereby the strength values of the structure of the bricks are significantly impaired.
AT 171428 B reports about the use of non-fired bricks made of non-acidic refractory material in rotary cement kilns, among other things, which bricks have compounds of magnesia with carbonic acid, as the result of CO2 treatment, as binders.
Currently, large-volume industrial furnaces for the production of cement, lime, magnesia, and doloma are lined with fired refractory MgO-based and/or CaO-based products in the form of fired bricks. These fired bricks are
magnesia chromite bricks,
magnesia spinel and spinel bricks,
magnesia zirconia and magnesia zircon bricks,
magnesia hercynite and magnesia galaxite bricks,
dolomite and dolomite-magnesia bricks,
forsterite and olivine bricks,
magnesia forsterite bricks,
magnesia pleonast bricks,
magnesia bricks.
(Gerald Routschka, Hartmut Wuthnow: Taschenbuch Feuerfeste Werkstoffe [Pocket Book of Refractory Materials], 4th edition, 2007, Vulkan-Verlag [publishing company], pages 171 to 185 and 197 to 235).
In the field of refractory products, a distinction is made, in terms of classification, between magnesia spinel bricks, which have at least 40 mass-% MgO, and spinel bricks, which contain more than 20 mass-% and less than 40 mass-% MgO. Sintered spinels and melted spinels serve as raw materials.
Within the scope of use according to one embodiment of the invention, the mineralogical compositions of hercynite, galaxite, pleonast are considered to be part of the spinel group.
In general, refractory bricks that are composed of granulations of refractory materials require binding for the grains of the granulation, in order to maintain the shape of the bricks. At room temperature, an organic binder, composed, for example, of lignin sulfonate or an acidic starch solution or also of synthetic resin, is generally used to shape alkaline refractory bricks, e.g. on the basis of MgO for the cement industry.
After shaping by means of pressing, the pressed brick is stable in shape, can be handled and set onto furnace carriages. Subsequently, drying takes place, in order to remove the liquid water, and afterwards firing takes place. During this firing, which takes place in oxidizing manner, the organic binder is burned off. It is true that binder residues can be detected maximally up to temperatures of approximately 1000° C., but the binding force is lost starting at approximately 400° C. Therefore one also speaks of a temporary binder or temporary binding. Nevertheless, the strength of the brick is sufficient to bring it through the firing process. Ceramic sintering that imparts the strength for use in a lining of an industrial furnace to the fired brick starts, depending on the refractory material, at approximately 900° C.; the sintering speed then increases with the temperature and is also influenced by the firing duration. After firing, complete ceramic binding is then present over the entire brick structure, due to the sintering; organic components in the binding can no longer be found, because the carbon has been completely combusted.
The known fired refractory products mentioned above, which are currently in use, have the disadvantage that they are very complicated to produce, because a firing process must take place after pressing. Aside from the fact that this firing process is very energy-intensive, a great number of defects, namely what are called firing defects, can come about. In this connection, cracks, lumps, drips, and deformations, for example, should be mentioned. In addition, the non-homogeneity of the temperature distribution in the firing kiln, which naturally occurs, leads to different properties of the bricks, even though the composition of the green bricks before firing is the same, in each instance. Thus, undesirable variations in the brick properties, e.g. the strength, porosity, elasticity can come about. Furthermore, the dimensional stability of the bricks cannot be sufficiently guaranteed from one brick to another, because a change in the shape of the bricks occurs during firing, due to different shrinkage.